Hyponatremia in patients with severe anorexia nervosa was associated with more severe and longer duration of disease.

Introduction: Several mechanisms are thought to contribute to hyponatremia in patients with anorexia nervosa (AN). The aims of this descriptive, cross-sectional study among patients admitted to a specialized somatic unit for eating disorders (ED) were to determine the frequency of hyponatremia and to compare medical findings between patients with normonatremia and hyponatremia. Methods: This retrospective, descriptive cross-sectional study included patients admitted to the unit between December 2016 and October 2021. Demographic, medical history, and clinical data were extracted from medical records. Patients were categorized according to plasma sodium concentration (<135 mmol/L vs. ≥135 mmol/L). Results: Among 131 patients, 17 (13%) had hyponatremia at admission. Hyponatremia was associated with lower BMI, lower nadir BMI, longer disease duration, and an adverse biochemical profile (lower albumin, higher creatinine, higher platelet counts, and higher bicarbonate levels). Thirteen patients (10%) were deceased at follow-up; hyponatremia was associated with mortality in unadjusted analysis (OR 8.03, 95% CI 2.29-28.16) but not after multivariable adjustment. Discussion: The study found that 13% of patients admitted to a specialized somatic unit for ED had hyponatremia, which clustered with indicators of more severe and longstanding AN (lower BMI, lower nadir BMI, longer disease duration). Hyponatremia was associated with mortality in unadjusted analyses, but this association was attenuated after adjustment for age and illness severity. Despite the established potential of purging to induce hyponatremia, our findings suggest that, in this severely ill inpatient population, overall illness severity and chronic medical deterioration may be more important determinants of both hyponatremia and mortality risk than purging per se.